Method for monitoring the performance of a nox trap

ABSTRACT

A method and apparatus for on-board monitoring of NO x  trap performance is proposed that uses two HEGO sensors, one positioned upstream of the NO x  trap, and the other positioned downstream of the NO x  trap. When the engine A/F is reduced from lean to stoichiometric or rich operation to regenerate or purge the NO x  trap the difference in the time it takes for the upstream and downstream HEGO sensors to switch from a lean to a rich indication provides a quantitative measure of the amount of NO x  that was stored on the NO x  trap during the previous lean period of operation. This measure is related to an estimated amount of NO x  produced by the engine to infer the operating performance or efficiency of the NO x  trap. The difference in the output voltage of the two sensors is compared with a predetermined value to determined when to terminate the NO x  purge. When the efficiency drops below a predetermined value the time that the engine is run in a lean cruise mode is reduced. If the time is reduced below a minimum time interval, a sulfur purge is performed. If sulfur purges are required more often than a predetermined repetition time, the lean cruise mode is terminated and an indicator lamp is energized.

TECHNICAL FIELD

This invention relates to monitoring the status and performance ofexhaust gas purification devices installed in the exhaust passage of aninternal combustion engine.

BACKGROUND ART

Presently, NO_(x) traps are considered a potential exhaustafter-treatment technology for lean burn engines. NO_(x) trap technologytypically utilizes alkali metal or alkaline earth materials incombination with platinum in order to store or occlude NO_(x) under leanoperating conditions. The mechanism for NO_(x) storage involves theoxidation of NO to NO₂ over the platinum followed by the subsequentformation of a nitrate complex with the alkaline metal or alkalineearth; under stoichiometric or rich conditions, the nitrate complexesare thermodynamically unstable, and the stored NO_(x) is released and iscatalytically reduced by the excess of CO, H₂, and HCs in the exhaust.

If the NO_(x) trap deteriorates over time, the ability to trappollutants degrades with resultant increase in atmospheric pollution.Therefore, it is desirable that NO_(x) trap technology implementedprovide an on-board computer driven diagnostic indication ofdeterioration or degradation of the NO_(x) trap beyond a predeterminedlimit.

SUMMARY OF THE INVENTION

In accordance with the present invention, a method and apparatus isprovided for making on-board measurements of NO_(x) trap sorption thatpermits vehicle on-board computer monitoring and evaluation of NO_(x)trap performance.

It has been found that during NO_(x) trap purging, the lean to richresponse time (T_(LR)) of a HEGO (Heated Exhaust Gas Oxygen) sensorpositioned downstream from the NO_(x) trap is reduced by an amount whichis proportional to the quantity of NO_(x) stored on the trap. As NO_(x)sorption efficiency increases, more NO_(x) is stored on the trap, andthe T_(LR) of the downstream HEGO sensor increases as well.

Based on the above discovery, the present invention proposes to use thistime interval between the initiation of the purge operation and theswitching of the downstream HEGO sensor as an indicator of the amount ofNO_(x) which was stored onto the NO_(x) trap during the previous leanperiod of operation. Also, this time delay is used in a diagnosticroutine for indicating degradation of the NO_(x) trap performance to anextent requiring attention by service personnel.

More particularly, in a preferred embodiment of the invention two HEGOsensors, one positioned upstream of the NO_(x) trap, the otherpositioned downstream of the NO_(x) trap are employed. When the engineA/F is reduced from lean to stoichiometric or rich operation toregenerate the NO_(x) trap (i.e. in order to remove the stored NO_(x)and subsequently convert it to N₂), the difference between T_(LR) forthe upstream and downstream HEGO sensors provides a quantitative measureof the amount of NO_(x) that was stored on the NO_(x) trap during theprevious lean period of operation. This estimation of the amount ofNO_(x) stored by the trap is related to a predicted amount of NO_(x)produced by the engine to infer the operating performance or efficiencyof the NO_(x) trap. Also, the output voltage signal differential betweenthe downstream and the upstream HEGO sensor is checked to determinedwhen to terminate the NO_(x) purge.

If trap sorption efficiency drops below a predetermined efficiency, thelean operation time is reduced in an attempt to improve efficiency. Ifand when the reduced lean time duration drops below a predeterminedminimum lean operation time, a sulfur purge of the trap is desirable andis performed.

If the interval between successive sulfur purges becomes less than apredetermined interval, this is indicative of deterioration of the trapbeyond that which can be remedied by the normal purging operations.Accordingly, the lean cruise mode of engine operation is terminated andoperation reverts to a closed loop stoichiometric mode and an indicatorlamp is energized, so that appropriate remedial action can be taken bythe operator.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the present invention may be had fromthe following detailed description which should be read in conjunctionwith the drawings in which:

FIG. 1 is an overall block diagram of the control system of the presentinvention;

FIGS. 2 and 3 graphically demonstrate the similar quantitativerelationship between %NO_(x) sorption efficiency and the lean to richswitch time of a downstream oxygen sensor over a range of temperature;

FIG. 4 shows that the lean to rich switch time of a downstream oxygensensor is substantially directly proportional to the amount of NO_(x)that is stored on the trap;

FIG. 5 is a flowchart depicting the conditions under which a lean cruisemode of engine operation is entered;

FIGS. 6a and 6b are timing diagrams showing the timing of the initiationand termination of the NO_(x) purge operation;

FIG. 7 is a flowchart depicting the conditions under which the timeinterval for lean mode is adjusted;

FIG. 8 is a flowchart depicting the conditions under which a sulfurpurge is carried out as well as the circumstances under which the leancruise mode is terminated and an indicator lamp is energized.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

Referring now to the drawings and initially to FIG. 1, a block diagramof the present invention is shown. A fuel pump 10 pumps fuel from a tank12 through a fuel line 14 to a set of injectors 16 which inject fuelinto an internal combustion engine 18. The fuel injectors 16 are ofconventional design and are positioned to inject fuel into theirassociated cylinder in precise quantities as determined by an electronicengine controller (EEC) 20. The fuel tank 12 contains liquid fuels, suchas gasoline, methanol or a combination of fuel types.

An exhaust system 22, comprising one or more exhaust pipes and anexhaust flange seen at 24, transports exhaust gas produced fromcombustion of an air/fuel mixture in the engine to a conventionalthree-way catalytic converter 26. The converter 26 contains catalystmaterial that chemically alters the exhaust gas to generate a catalyzedexhaust gas. A heated exhaust gas oxygen (HEGO) sensor 28, detects theoxygen content of the exhaust gas generated by the engine 18, andtransmits a representative signal over conductor 30 to the EEC 20. ANO_(x) trap 32 is located downstream of the converter 26 for trappingnitric oxide contained in the exhaust gas exiting the converter. A HEGOsensor 34 detects the oxygen content of the exhaust gas upstream of thetrap 28 while a HEGO sensor 36 detects the oxygen content of the exhaustgas downstream of the trap 28. The sensor 34 and 36 transmits signalsover respective conductors 38 and 40 to the EEC 20.

Still other sensors, indicated generally at 46, provide additionalinformation about engine performance to the EEC 20, such as crankshaftposition, angular velocity, throttle position, air temperature, etc.over conductor 50. The information from these sensors is used by the EEC20 to control engine operation.

A mass air flow sensor 48 positioned at the air intake of engine 18detects the amount of air inducted into an induction system of theengine and supplies an air flow signal over conductor 52 to the EEC 20.The air flow signal is utilized by EEC 20 to calculate a value that isindicative of the air mass flowing into the induction system inlbs./min.

The EEC 20 comprises a microcomputer including a central processor unit(CPU) 54, read only memory (ROM) 56 for storing control programs, randomaccess memory (RAM) 58, for temporary data storage which may also beused for counters or timers, and keep-alive memory (KAM) 60 for storinglearned values. Data is input and output over I/O ports generallyindicated at 62, and communicated internally over a conventional databus generally indicated at 64. The EEC 20 transmits a fuel injectorsignal to the injectors 16 via signal line 64. The fuel injector signalis varied over time by EEC 20 to maintain an air/fuel ratio determinedby the EEC 20. An indicator lamp generally indicated at 66 is controlledby the EEC 20 to provide an indication of the condition of the NO_(x)trap 32 as determined by input data from the various sensors asdescribed more fully hereinafter.

The program stored in ROM 58 implements an air/fuel strategy where theengine is operated in lean mode or relatively high air to fuel ratio(A/F) for fuel economy under certain engine speed/load conditions.During the lean mode, NO_(x) and SO_(x) accumulates in the NO_(x) trap.After predetermined criteria are met, indicative of substantially totalsorption of the trap 32, the A/F is switched to a relatively richmixture to purge the trap of NO_(x). After the purge node is completedthe EEC returns to the lean mode of operation. Alternatively, the EECprogram may call for a stoichiometric mode of operation instead of therich mode for purging the trap of NO_(x).

Referring now FIGS. 2, 3, and 4, the relationship between lean to richswitch time (T_(LR)) of a HEGO sensor placed downstream of a NO_(x) trapand the quantity of NO_(x) stored on the trap is graphicallyillustrated. FIGS. 2 and 3 contrast NO_(x) trap sorption efficiency as afunction of temperature to the corresponding lean to rich switch time(T_(LR)) of a HEGO sensor placed downstream of the NO_(x) trap. TheNO_(x) trap sorption efficiency and the downstream HEGO's T_(LR) exhibitvery similar qualitative behaviors. As NO_(x) sorption efficiencyincreases, more NO_(x) is stored on the trap, and the T_(LR) of thedownstream HEGO sensor increases as well.

FIG. 2 shows the average NO_(x) sorption efficiency as a function oftemperature during a 5 minute lean cycle for a conventional strontiumbased NO_(x) trap. With increasing temperature, NO_(x) sorptionefficiency first increases, reaches a maximum level at approximately300°-350° C., and then decreases. These measurements were made in alaboratory flow reactor with a simulated exhaust gas consisting of 10%H₂O, 10% CO₂, 500ppm NO_(x), 7%O₂, in a balance of N₂. To purge orregenerate the NO_(x) trap, the O₂ in the exhaust gas was turned off andreplaced with 0.58% CO. The space velocity was 30,000 hr-1.

FIG. 3 shown a graph of the corresponding lean to rich switch time(T_(LR)) for a conventional Exhaust Gas Oxygen Sensor (EGO) placeddownstream of the NO_(x) trap. T_(LR) is defined as the time periodbetween the initiation of the NO_(x) trap purge and the observation of aminimum 0.5 volt sensor output signal. The NO_(x) trap sorptionefficiency and T_(LR) display very similar qualitative behaviors. AsNO_(x) trap sorption efficiency increases, more NO_(x) is stored ontothe trap, and the T_(LR) of the downstream EGO sensor increases as well.It is believed that the NO_(x) which is stored onto the trap behavesvery much like stored oxygen and simply reacts with the CO and H₂ in theexhaust during purging hence delaying rich breakthrough.

FIG. 4 shows a graph of NO_(x) storage as a function of T_(LR) at 350°C. The lean operating period was varied in order to vary the quantity ofNO_(x) stored onto the trap. At a given temperature, the T_(LR) which isobserved during purging of the NO_(x) trap is seen to be directlyproportional to the quantity of NO_(x) which was stored onto the trapduring the previous lean period of operation. The present inventionutilizes this relationship between NO_(x) sorption and T_(LR) to controltrap purge time, to determine whether the time interval of leanoperation should be reduced, and to determine when the trap should bereplaced. Also, this relationship is used to determine when to desulfatethe trap to rid the NO_(x) trap of SO_(x).

Referring now to FIG. 5, a flowchart depicting the criteria for enteringthe lean cruise mode of operation is shown. The lean cruise mode ofoperation includes an open loop fuel control mode where the engine isoperated with a lean fuel mixture of for example 20 parts air to 1 partfuel. The lean cruise mode of operation also includes a closed loop fuelcontrol mode, which is periodically entered from the open loop mode,where the engine is operated at a stoichiometric air fuel ratio of about14.5 to 1 for a time interval sufficient to purge the NO_(x) trap ofNO_(x) prior to return to the lean mode. There is a flag LCFLG thatreflects the status of the lean cruise mode. While in the lean cruisemode, the engine is normally operating in an open loop lean mode and isperiodically placed in a closed loop stoichiometric mode or slightlyopen loop rich mode for purging the NO_(x).

At block 70, an indicator lamp flag LAMFLG is checked. This flag is setwhenever the EEC 20 determines that the NO_(x) trap has degraded to apoint where the normal SO_(x) purging operations are no longersufficient and the NO_(x) trap requires further attention and may needto be replaced. Such a condition would be indicated to the vehicleoperator by the energized state of the indicator lamp 66 and theoccurrence of NO_(x) degradation would be logged in the keep-alivememory 60. IF LAMFLG is reset (0), indicating normal NO_(x) trapoperation, then at block 72 the air mass inducted into the engine, aswell as other engine operating conditions, such as speed and enginecoolant temperature, are measured to determined the proper engine airfuel ratio (A/F). If degradation of the NO_(x) trap has occurred(LAMFLG=1), or if conditions are such that lean operation is notdesirable, as determined by the decision block 74, then a lean cruiseflag LCFLG is reset (0) at block 76 and the subroutine returns to themain program. Otherwise, the lean cruise flag LCFLG is set (1) at block78 and the subroutine returns to the main program. The lean cruise modeof operation includes operating at a lean A/F for a length of time T₁,during which time the engine speed and load are used to estimate thecumulative amount of NO_(x) produced by the engine. After the timeinterval T₁ has expired, a purge of the NO_(x) trap is performed byoperating the engine at a relatively rich A/F for a purge intervalbefore returning to the relatively lean operation.

A timing diagram of the NO_(x) purge operation is shown in FIGS. 6a and6b. FIG. 6a shows an air/fuel ratio schedule as a function of time,while the engine is operation in a lean cruise mode of operation at anopen loop air/fuel ratio of 20. When lean time LT becomes greater thanT1 a purge of the trap 32 is appropriate so the air/fuel ratio isstepped from a lean value to a slightly rich value, where an air/fuelratio of 14.5 represents stoichiometry. When this occurs the upstreamsensor 34 switches immediately from low voltage to a high voltage, asshown in FIG. 6b. As indication by the dotted line, the switching of thedownstream sensor is delayed by the amount TD. The time delay requiredfor the downstream sensor 36 to reach a predetermined voltage, forexample, one-half volt as shown in FIG. 6b is measured (block 100). Whenthe output voltage difference between the downstream sensor 36 andupstream sensor 34 reaches a predetermined value S_(c) (block 108) theNO_(x) purge is terminated and lean operation is resumed.

Referring now to FIG. 7, a flowchart depicting the criteria for purgingthe NO_(x) trap and the calculation of NO_(x) storage efficiency, isshown. At block 86, LCFLG is checked to determined if the system isbeing operated in a lean cruise mode. If not, the routine returns to themain program. If so, a sulfur purge flag SPFLG is checked at decisionblock 88. If SPFLG is set (1), then a sulfur purge of the trap isinitiated as will be described hereinafter. If SPFLG is reset (0), thetime duration of the lean mode of operation T₁, is compared with apredetermined minimum time period T_(1c). Unless T₁ is greater than thispredetermined time interval T_(1c), lean cruise operation may need to beterminated. The time interval T₁ is initially a predetermined value andwill remain so as long as NO_(x) trap storage efficiency remains above apredetermined or required efficiency value, but T₁ will be reduced asexplained below in order to maintain the required efficiency. If it isdetermined at block 90 that T₁ is not greater than the predeterminedtime period T_(1c), this may indicate that the NO_(x) trap isdeteriorated due to the adsorption of SO_(x), an undesirable butunavoidable process. Accordingly, the sulfur purge flag is set and thelean and NO_(x) flags are reset at block 120 and the operation returnsto the main program. The next time through this routine a sulfur purgewill be called for at the decision block 88.

If T₁ is greater than T_(1c), then the conditions of a NO_(x) purge flag(NPFLG), is checked at decision block 92. If the NO_(x) purge flag isreset, that is, the engine is operating in a lean mode, lean time LT isincremented at block 94 and compared to T₁ at block 96. If the lean timeis not greater than the predetermined time period for lean operation asdetermined by the block 96, then an estimate of the amount of NO_(x)which has been introduced to the trap since the last purge is made inblock 116. SUM N^(e0), determined in block 116, is a prediction ofcumulative NO_(x) produced by the engine based on air mass inducted intothe engine and engine speed as input from block 118. If, on the otherhand, the measured lean operating time is greater than the set timeperiod for lean operation T₁, as determined at block 96, the NPFLG flagis set as shown at block 98 and the NO_(x) purge operation is begun byswitching from a lean mode to a relatively rich A/F. The next timethrough the loop at block 92 the NO path will be taken.

During the NO_(x) purge, the time delay that occurs between switching ofthe front and rear EGO sensors, due to NO_(x) accumulation, is measuredat block 100. Based on this time delay, the amount of NO_(x) stored onthe trap N_(s) is determined at block 102 as a function of the traptemperature (FIG. 4), which is input from block 104. The traptemperature may be obtained in several known ways such as from atemperature sensor or based on sensed air mass or estimated by way ofanother input.

The NO_(x) storage efficiency S_(eff) is determined at block 106 basedon the ratio N_(s) /SUM N^(e0). In other words, storage efficiency isratio of the amount of NO_(x) stored in the trap to the amount of NO_(x)generated by the engine. At decision block 108, the voltage S2, of thedownstream HEGO sensor 36 is subtracted from the voltage S1, of theupstream HEGO sensor 34 and the difference is compared to apredetermined difference S_(c) to determine whether it is time toterminate the NO_(x) purge. As soon as the difference drops below thepredetermined difference value, the purge may be considered complete andis terminated and the NO_(x) purge flag NPFLG is reset (0), the leancruise time counter or timer LT is reset, and the predicted NO_(x) valueSUM N^(e0) is reset at block 110.

If the NO_(x) storage efficiency is less than a predetermined NO_(x)storage efficiency SC_(eff) as determined by the block 112, then thetime period for lean operation, T₁, is reduced toward T_(1c), by apredetermined amount, at block 114. If the lean time interval has beenreduced below the predetermined time period T_(1c) as determined by theblock 90 then the sulfur purge flag (SPFLG) is set as indicated in theblock 120. With SPFLG=1, the next time through this routine a sulfurpurge will be called for at the decision block 88.

Referring now to FIG. 8, the subroutine for carrying out a sulfur purgeand on-board NO_(x) trap diagnostics is shown. Sulfur purge isaccomplished by raising the NO_(x) trap temperature to a predeterminedlevel, for example above 550 degrees C., while exposing the NO_(x) trapto a rich exhaust gas mixture. Additional air from a separate air supplyand pump may be introduced under EEC control to achieve desired traptemperature in order to generate an exotherm on the NO_(x) trap 32 andhence achieve the desired temperature.

If the lean cruise flag (LCFLG) is set (1), and the sulfur purge flag(SPFLG) is set (1), as determined by blocks 86 and 88 of FIG. 6, asulfur purge is initiated at block 124, unless the time period betweensuccessive sulfur purges (TSP) is less than a predetermined time period(TSP_(c)) as determined by the block 122. At block 126 the time sincethe last sulfur purge (TSP) is calculated. When the purge is completedas determined by block 128, the sulfur purge flag (SPFLG) is reset (0)at block 130 and the subroutine returns to the main program. Completionof sulfur purge would be based on the trap 32 being above a thresholdtemperature for a predetermined time period or on other criteria. On theother hand, if the time period between sulfur purges is less than thepredetermined time period TSP_(c), this frequent need to perform aSO_(x) purge is an indication that the trap is not being properly purgedand may be defective. In this event the system reverts to astoichiometric operation at block 132, the indicator lamp is energizedat 134, and the associated flag (LAMFLG) is set at 136. This will causethe lean cruise flag LCFLG to be reset (0) at block 76 (FIG. 5) the nexttime a decision is called for at block 70. Thus, a diagnostic lamp isenergized whenever the NO_(x) trap exhibits an apparent permanent lossin activity which is not alleviated by the NO_(x) and SO_(x) purgingoperations normally intended to revitalize the trap.

While two HEGO sensors 34 and 36 are shown, the sensor 34 could beeliminated. In this particular case, the time interval measured at block100 would be simply the time delay between the initiation of the NO_(x)purge (switching engine A/F ratio from lean to rich or stoichiometric)and the lean to rich switch of the rear HEGO sensor 36. Also, a minimumoutput signal or voltage of sensor 36 would be checked at block 108 todetermine that an adequate NO_(x) purging had been completed. Further,the NO_(x) purging operation may commence based on other criteria than apredetermined time interval in lean mode. This change would involve amodification of the operations performed at blocks 90, 96 and 114 toreflect the new criteria.

While the best mode for carrying out the present invention has beendescribed in detail, those familiar with the art to which this inventionrelates will recognize various alternative designs and embodiments forpracticing the invention as defined by the following claims.

What is claimed is:
 1. A vehicle on-board computer method of diagnosingthe status of a NO_(x) trap disposed in an exhaust passage of aninternal combustion engine, comprising a sequence of the followingsteps:performing a purge of said trap after meeting predetermined leanmode engine operating criteria; and providing an indication of trapdeterioration if the time interval between successive purges is lessthan a predetermined time interval.
 2. The invention defined in claim 1wherein said purge is a SO_(x) purge.
 3. The invention defined in claim2 including the further step of performing a NO_(x) purge prior to saidSO_(x) purge.
 4. The invention defined in claim 3 wherein said NO_(x)purge is initiated periodically during a lean cruise mode of engineoperation and the time interval of lean operation is adjusted to improvetrap sorption efficiency whenever trap sorption efficiency drops below apredetermined trap sorption efficiency.
 5. The invention defined inclaim 3 wherein said SO_(x) purge is initiated whenever said timeinterval of lean operation drops below a predetermined time interval. 6.The invention defined in claim 5 wherein said indicator is energizedwhenever the time interval between SO_(x) purges drops below apredetermined time interval.
 7. A method of diagnosing the status of aNO_(x) trap disposed in an exhaust passage of an internal combustionengine, comprising a sequence of the following steps:performing a NO_(x)purge on said trap after operating said engine in a lean mode ofoperation; determining the sorption efficiency of said trap; reducingthe time duration of said lean mode of operation if said efficiencydrops below a predetermined level; performing a SO_(x) purge on saidtrap if said time duration falls below a predetermined minimum timeinterval; and providing an indication of trap deterioration if the timeinterval between SO_(x) purges is less than a predetermined timeinterval.
 8. The method defined in claim 7 wherein the NO_(x) purge isinitiated after operating the engine in said lean mode for apredetermined time interval and wherein the NO_(x) purge is terminatedbased on the oxygen content of the exhaust downstream from said trap. 9.The method defined in claim 8 wherein the sorption efficiency iscomputed by:determining the amount of NO_(x) stored on the trap;estimating the amount of NO_(x) produced by the engine during the lastlean mode of operation; and dividing the amount of NO_(x) stored on thetrap by the estimated amount of NO_(x) produced by the engine.
 10. Themethod defined in claim 9 wherein the determination of the amount ofNO_(x) stored on the trap is based on the time interval betweeninitiation of said NO_(x) purge and detection of a stoichiometricexhaust gas condition downstream of the trap.
 11. A vehicle on-boardcomputer diagnostic system for monitoring the status of a NO_(x) trapdisposed in an exhaust passage of an internal combustion enginecomprising:a computer for generating a command to purge said NO_(x)trap; a sensor disposed downstream from said trap; an indicator; saidcomputer generating a command to energize said indicator based on thesorption efficiency of said trap.
 12. The system of claim 11 whereinsaid sensor is an exhaust gas oxygen sensor.
 13. The system of claim 12wherein said computer generates a NO_(x) purge command causing a changein the mixture supplied to said engine from a relatively lean A/F to arelatively rich A/F.
 14. The system of claim 13 wherein said sorptionefficiency is the ratio of the amount of NO_(x) stored on the trapdivided by the amount of NO_(x) generated by the engine.
 15. The systemof claim 14 wherein the amount of NO_(x) stored on the trap is afunction of the trap temperature and the lean to rich response time ofsaid sensor.
 16. The system of claim 15 wherein the amount of NO_(x)generated by the engine is a function of engine speed and air massinducted into said engine.
 17. The system of claim 16 wherein saidcomputer increases the frequency of NO_(x) purges if the sorptionefficiency drops below a predetermined efficiency.
 18. The system ofclaim 17 wherein said computer initiates a SO_(x) purge if the timebetween NO_(x) purges drops below a predetermined time interval.
 19. Thesystem of claim 18 wherein said indicator is energized if the timeinterval between SO_(x) purges drop below a predetermined time interval.20. The system of claim 19 comprising a second exhaust gas oxygen sensorlocated upstream of said NO_(x) trap and said NO_(x) purge is terminatedwhen the difference in the output of the downstream sensor and theupstream sensor is less than a predetermined threshold.